The present application is a continuation of International Patent Application No. PCT/EP2004/008092, filed Jul. 17, 2004, and claims priority under 35 U.S.C. § 119 to German Application No. 103 33 654.0-51, filed Jul. 24, 2003. The entire disclosure of the aforementioned documents is herein expressly incorporated by reference.
The present invention relates to a control device for an at least partially four-wheel-driven motor vehicle. More particularly, the present invention relates to a control device for an at least partially four-wheel-driven motor vehicle, having a control unit which can variably distribute the driving torque of a drive unit as a function of operating conditions, to primary driving wheels, which are permanently connected with the drive unit, and to secondary driving wheels which, if required, can be connected by way of a transfer clutch with the drive unit, in that the control unit determines a desired clutch torque which is to be set by an actuator device at the transfer clutch.
A control device of this type is described, for example, in German Patent Document DE 100 54 023 A1. Accordingly, a torque distribution device is known for changing the torque distribution ratio between the wheels of a front axle and the wheels of a rear axle by a corresponding control of a friction clutch as a longitudinal blocking device (transfer clutch). The driving action of a vehicle can be influenced considerably by the determination of a torque distribution ratio. In particular, the object of German Patent Document DE 100 54 023 A1 relates to the driving dynamics when cornering. In this case, a temporarily four-wheel-driven motor vehicle may be a basically front-wheel-driven motor vehicle with a rear-wheel drive which can be connected by means of a transfer clutch; a basically rear-wheel-driven motor vehicle with a front-wheel drive which can be connected by way of a transfer clutch; or a permanent all-wheel drive vehicle with a controllable transfer clutch for changing the torque distribution between the front and rear axle.
Generally, as described below, primary driving wheels are the wheels that are permanently connected with the drive unit, and secondary driving wheels are the wheels that, if required, can be connected with the drive unit by way of the transfer clutch.
Furthermore, such known control systems usually have a control of the transfer clutch as a function of the rotational speed difference between a rotational speed of the primary driving axle and the rotational speed of the secondary driving axle (for example, German Patent Document DE 34 27725 C2).
It is an object of the present invention to improve a control system of the previously mentioned type with respect to the protection of components.
The present invention is based on the fact that, particularly for reasons of driving dynamics, the desired clutch torque should always be reduced as the vehicle speed increases. However, the considerations on which the invention is based take into account more than merely the driving dynamics and the driver's intention. In the case of any mechanical construction, with a view to the dimensioning of components, a compromise is required between the saving of cost and weight, on the one hand, and the stability, on the other hand. The object of the invention relates particularly to the differentials at the driving axles as well as the drive shafts as the components to be protected.
With respect to driving dynamics, the prevention of twisting is important at high speeds. Accordingly, at high speeds, the entire driving torque should basically be transferred to the primary driving wheels; the desired clutch torque should therefore be zero. With respect to wear, however, such a control would result in an increased temperature-caused stress as the dimensioning of the components decreases. This problem is solved by the part of the control device, which relates to a defined high-speed range, according to the present invention.
Particularly during the starting operation, with respect to the driving dynamics, the maximizing of the traction is important at very low speeds. When the driving torque is distributed corresponding to the axle load distribution of a vehicle, maximal traction is achieved. Naturally, an approximately uniform distribution is advantageous in this case, which automatically results in the best component protection. According to the invention, an unlimited traction optimization therefore takes place in a defined speed-related starting range.
At speeds in the normal range (particularly partial load), a distribution of the torque which optimizes the component stability is particularly advantageous, because most driving time takes place in the normal range. That is, according to the invention, a distribution of the driving torque is achieved, and, thus, a durable component protection is ensured (resistance to fatigue in the case of long-duration strain). In this case, the distribution of the driving torque has to be adapted approximately to the most unfavorably dimensioned component. Also in the normal range, a distribution which minimizes twisting would be more desirable. This would require a distribution of the entire driving torque to the primary drive shaft or driving axle. If, however, the components of the primary driving axle (particularly its differential) are not designed for such a long-term operation, a distribution of the driving torque corresponding to the fraction of the driving torque to be withstood for a long duration without destruction has to be defined.
The speed-range-related clutch torques assigned to the respective speed ranges basically comprise constant values as well as torque courses.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.